In a hologram recording and reproducing method, in particular, in a hologram recording and reproducing method in the area of an optical storage system, intensity modulation is caused to be performed on a signal beam so that a pattern arrangement of bits of 1 (for example, light intensity=high) and bits of 0 (for example, light intensity=low) can be obtained by using a spatial light intensity modulator (also simply referred to as an intensity modulator), such as, for example, a transmission-type liquid crystal panel or a DMD (Digital Micromirror Device: registered trademark), as a spatial light modulator. In addition, a beam (called a signal beam) generated by performing intensity modulation as described above and a reference beam, which is different from the signal beam, are caused to be applied to a hologram recording medium so that data recording is performed by using interference fringes between the signal beam and the reference beam (Japanese Unexamined Patent Application Publication No. 2007-79438).
As such a hologram recording and reproducing method, a coaxial method for arranging a signal beam and a reference beam along the same axis and applying the signal beam and the reference beam to a medium exists.
FIG. 18 shows an example of the configuration of a hologram recording and reproducing device employing a conventional coaxial method. Note that, in FIG. 18, only the configuration of an optical system of the recording and reproducing device is mainly extracted and shown and the other portions are omitted.
In FIG. 18, the configuration of a recording and reproducing device that corresponds to a reflection-type hologram recording medium provided with a reflection film, as a hologram recording medium, is exemplified.
First, a laser diode (LD) 101 is provided as a light source, in order to acquire laser beams for recording and reproduction. As the laser diode 101, for example, a laser diode provided with an external resonator is adopted, and the wavelength of the laser beam is set, for example, to 410 nanometers.
Emission light from the laser diode 101 passes through a collimator lens 102, and then is made incident on an SLM intensity modulator 103.
The intensity modulator 103 is constituted by, for example, a transmission-type liquid crystal panel or the like. The intensity modulator 103 is configured such that each picture element is controlled to be driven in accordance with a driving signal from a driving circuit, which is not illustrated, and the intensity modulator 103 thus performs, on incident light, spatial light intensity modulation (also simply referred to as intensity modulation) corresponding to recording data. Specifically, the intensity modulator 103 is configured so as to be capable of performing ON/OFF control of light for each picture element (for each pixel) in such a manner that, for example, a picture element that is turned ON in accordance with a driving signal transmits incident light and a picture element that is turned OFF in accordance with a driving signal does not transmit incident light. Due to such ON/OFF control by the intensity modulator 103, it is configured such that data of “0” or “1” can be recorded for each picture element (pixel).
Light that has been subjected to spatial light modulation by the intensity modulator 103 passes through a polarizing beam splitter 104, and then passes through a relay lens optical system constituted by a relay lens 105→a light-shielding mask 106→a relay lens 107. Furthermore, after passing through a ¼-wavelength plate 108, the light is converged onto an objective lens 109 and is applied to a hologram recording medium 110.
Here, at the time of recording, a signal beam that has been subjected to spatial light intensity modulation corresponding to recording data by the intensity modulator 103 as described later and, for example, a ring-shaped reference beam that is concentric with respect to the signal beam are generated. That is, the signal beam and the reference beam generated as described above are caused to be converged onto the hologram recording medium 110 through the path explained above.
Meanwhile, at the time of reproduction, light from the laser diode 101 is made incident on the intensity modulator 103 through the collimator lens 102, similarly to the time of recording. The intensity modulator 103 is configured to perform intensity modulation for reproduction on the incident light to generate only a reference beam, at the time of reproduction. That is, at the time of reproduction, it is configured such that a signal beam is not generated and only a reference beam is applied to the hologram recording medium 110.
In accordance with application of the reference beam, diffracted light corresponding to recorded data on the hologram recording medium 110 is acquired, as described later. The diffracted light, serving as reflected light from the hologram recording medium 110, passes through the objective lens 109, and then is guided to the polarizing beam splitter 104 through the ¼-wavelength plate 108→the relay lens 107→the light-shielding mask 106→the relay lens 105. The reflected light guided through the above-mentioned path from the hologram recording medium 110 is reflected in the polarizing beam splitter 104, and the reflected light is guided to an image sensor 111, as shown in the figure.
The image sensor 111 includes an imaging element, such as, for example, a CCD (Charge Coupled Device) sensor or a CMOS (Complementary Oxide Semiconductor) sensor. The image sensor 111 receives the reflected light (diffracted light) guided as described above from the hologram recording medium 110, and converts the reflected light into an electric signal.
FIG. 19 and FIG. 20, which will be provided next, are illustrations for explaining basic operations of hologram recording and reproduction implemented by the configuration of the optical system explained above. FIG. 19 and FIG. 20 show operations at the time of recording and at the time of reproduction, respectively.
Note that, in FIG. 19, only the intensity modulator 103 and the objective lens 109 of the optical system shown in FIG. 18 are extracted and shown. In addition, in FIG. 20, in part (a) of FIG. 20, similarly, only the intensity modulator 103 and the objective lens 109 are shown, and in part (b) of FIG. 20, only the objective lens 109 and the image sensor 111 are extracted and shown.
First, at the time of recording shown in FIG. 19, the intensity modulator 103 performs intensity modulation on incident light so that the above-described reference beam and a beam (signal beam) having a light intensity pattern provided thereto based on a data pattern using “0” and “1” on the basis of recording data are arranged concentrically with respect to each other.
The light that has been subjected to intensity modulation (that is, the reference beam and the signal beam) is converged onto the hologram recording medium 110 through the objective lens 109, and the thus formed interference fringes between the reference beam and the signal beam are recorded as data onto the hologram recording medium 110.
In addition, at the time of reproduction, first, as shown in part (a) of FIG. 20, only a reference beam is generated by intensity modulation on incident light performed by the intensity modulator 103 and the reference beam is caused to be converged onto the hologram recording medium 110. On this occasion, the converged light is diffracted by interference fringes corresponding to a data pattern recorded on the hologram recording medium 110 and is output as reflected light from the hologram recording medium 110. That is, the diffracted light has a light intensity pattern reflecting recorded data, as shown in part (b) of FIG. 20, and it is configured such that data reproduction is performed on the basis of a result of detection of an intensity pattern of the diffracted light by the image sensor 111.
Here, as described above, the intensity modulator 103 is configured to generate a reference beam and/or a signal beam in accordance with the time of recording/reproduction. Thus, in the intensity modulator 103, a reference beam area A1, a signal beam area A2, and a gap area A3 are set, as shown in FIG. 21, which will be provided next. That is, as shown in FIG. 21, an area having a specific round shape including a central part of the intensity modulator 103 is defined as the signal beam area A2. In addition, for a peripheral portion of the signal beam area A2, across the gap area A3, the reference beam area A1 having a ring shape that is concentric with respect to the signal beam area A2 is defined.
In addition, the above-mentioned gap area A3 is defined as an area for allowing a reference beam to avoid becoming noise due to leakage into the signal beam area A2.
At the time of recording, by setting predetermined picture elements within the reference beam area A1 to “1” (light intensity=high), setting the other picture elements within the reference beam area A1 to “0” (light intensity=low), setting all of the gap area A3 and an area that is more outward than the reference beam area A1 to “0”, and causing each picture element within the signal beam area A2 to have a pattern of “0” or “1” corresponding to recording data, a reference beam and a signal beam shown in FIG. 19, which has been provided above, can be generated and output.
In addition, at the time of reproduction, by causing the reference beam area A1 to have the same pattern of “0” and “1” as that at the time of recording and setting all the other areas to bits of “0”, only a reference beam can be generated and output, as shown in part (a) of FIG. 20.
As described above, in the conventional hologram recording and reproducing method, the intensity of light is controlled in accordance with recording data to generate a signal beam, and data recording onto the hologram recording medium 110 is performed.
However, it is known that not only information on amplitudes but also information on phases can be recorded onto a hologram recording medium on which data recording is performed by using interference fringes by interference of two beams, a reference beam and a signal beam. In the present situation, it is configured such that recording data is recorded in accordance with information on light intensity by performing only modulation of light intensity corresponding to the recording data as described above and recorded data is reproduced, at the time of reproduction, by reading only recorded information on the light intensity. That is, as is clear from the above description, the fact that phase information can be recorded is not effectively utilized.